Format Exception

The below snipets are from my Form1 code. I keep getting a Format Exception because of my MessageBox.Show(…); declaration in my Stop() method. Why? What am I doing wrong? ...

    private TimeSpan iterationDuration = TimeSpan.Zero;
    ...

    public void Stop()
    {
        IsGameOver = true;
        MessageBox.Show(String.Format("Game Over\n\nScore = {0}\n\n Time Duration =   {l}", score, iterationDuration));
        Application.Exit();
   开发者_JAVA百科 }


    public void Start()
    {

        score = 0;
        IsGameOver = false;

        currentRedLightX = 0;
        currentRedLightY = 0;

        currentGreenLightX = width / 2;
        currentGreenLightY = height / 2;


        double minIterationDuration = SPEED; // 50 frames / sec

        //game loop
        while (!IsGameOver)
        {
            if (IsCollision())
            {
                score += 10;
            }

            DateTime startIterationTime = System.DateTime.Now;
            UpdateGameState();
            Render();
            DateTime endIterationTime = System.DateTime.Now;
            TimeSpan iterationDuration = endIterationTime - startIterationTime;
            if (iterationDuration.TotalMilliseconds < minIterationDuration)
                Thread.Sleep(Convert.ToInt32(minIterationDuration - iterationDuration.TotalMilliseconds));
            Application.DoEvents();
        }

    }

That's an {l} (lower case L) not a 1...